The present invention relates to a temperature- and humidity-controlled test chamber and a method of controlling the temperature and humidity thereof.
General purpose environmental test chambers typically are designed for several tasks requiring distinct modes of operation. One such task may be high and low temperature transitions and stabilizations with the temperature ranging from 180° C. to −70° C. Typically, to reach lower temperatures with mechanical refrigeration, a cascade refrigeration system is used. This requires two separate refrigeration circuits (stages) with a high pressure refrigerant in the low stage and a relatively lower pressure refrigerant in the high stage to “cascade” the heat out of the chamber, lowering the air temperature in the enclosed space.
Another task may be the precise control of temperature and humidity within the cabinet workspace. When operating in the temperature/humidity mode, it is important to keep the cooling coil above the freezing point of water to prevent excessive moisture migration (i.e., ice formation on the coil) and blockage of air flow through the cooling coil. To account for this, some designs incorporate a separate cooling coil within the chamber workspace and utilize the high stage refrigerant to maintain a cooling coil temperature above the freezing point of water. The refrigerant is expanded from a liquid to a vapor at a controlled pressure. The evaporating pressure is set based on the lowest temperature required for the temperature/humidity mode of operation, but above the freezing point of water. When cooling is required at the highest temperature/humidity combination in the operational range, a portion of the cooling coil temperature is significantly below the dew point of the air stream within the chamber, resulting in condensation and a considerable cooling requirement due to the latent heat of condensation. Moisture condensed from the air must be replaced to maintain the controlled humidity condition. Steam may be added by a boiler (not shown) that is open to the chamber atmosphere, or by pressurized steam rails (not shown). Moisture may also be added to the chamber by way of an atomizing spraying system. The re-introduction of moisture is often accompanied by sensible heat (steam), further increasing the cooling load. Additional cooling causes additional condensation, which increases the amount of steam required to replace the condensed moisture. As a result, temperature and humidity must be continuously monitored and corrected to ensure they stay within the desired ranges.
There is also a need in the market to operate at high temperature/humidity conditions while a product(s) within the chamber generates heat. A product, or thermal load, within the chamber may fall into one of two categories: a thermal load that generates heat is called a “live load,” and a thermal load that does not generate heat is called a “dead load.” Maintaining high temperature/humidity conditions in a system containing a live load is a challenge. The current systems either limit the temperature/humidity range, limit the allowable amount of heat dissipation by the live load, or are specialized such that the overall utility of the equipment is compromised.